In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. Upon removal, this spent nuclear fuel (“SNF”) is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. In order to protect the environment from radiation exposure, SNF is first placed in a canister, which is typically a hermetically sealed canister that creates a confinement boundary about the SNF. The loaded canister is then transported and stored in a large cylindrical container called a cask. Generally, a transfer cask is used to transport spent nuclear fuel from location to location while a storage cask is used to store SNF for a determined period of time. Such storage casks may be vertically oriented or horizontally oriented.
The decay heat generated in a canister in a typical ventilated storage module is rejected to the environment by the air entering the storage space near the bottom and exiting near the top. The upward flow of the ventilation air is actuated and sustained by the heat delivered from the SNF convectively raising its temperature as it rises inside the module. Regardless of the flow configuration, as the air heats up during its upwards movement in the storage module cavity, it becomes lighter in density and its relative humidity decreases, i.e., it becomes drier.
Stress Corrosion Cracking (SCC) of stainless steel nuclear waste canisters and containers in storage at costal sites with harsh marine environments is an important issue receiving increased industry and regulatory scrutiny. Canister designers and manufactures take preventative measures to minimize the chance of SCC developing by maintaining controlled temperatures during welding processes and engineering large conservative margins into canisters to keep stresses at a minimum. Investigations on SCC have demonstrated that SCC has a strong dependence on the surface temperature of the stainless steel canister. The dependence on the surface temperature is driven by the mechanism of the deposit of airborne contaminants (e.g. chlorides) and subsequent deliquesce of those contaminants on the stainless steel surface. It is known that dry air (defined as its relative humidity below 20%) cannot cause stress corrosion cracking. A higher surface temperature decreases the relative humidity of the air adjacent to the surface and prevents deliquesce the contaminants and subsequent penetration into the stainless steel surface, a precursor for SCC.
This means that if there is a sufficient amount of decay heat available, only a short lowermost region of the vertical canister and the bottom half of the horizontal canister are vulnerable to Stress Corrosion Cracking (hereinafter, “SCC”); the balance of the canister is not. The limited vulnerable region can be protected from SCC by other means such as peening. The problem arises, however, when the decay heat progressively declines with the passage of time, the heating of air becomes much slower making a greater portion of the canister vulnerable to SCC.
Thus, a need exists to reduce or prevent the risk of stress corrosion from spreading over the surface of a horizontal or vertical canister.